This research investigates a design and development approach to improving science teachers' access to effective professional development (PD) in a fully online, asynchronous environment. Working with a small number of teachers, this study explores how a design combining social capital mechanisms with essential teacher learning and PD characteristics supported teachers' abilities to participate in the online course and collaboratively build knowledge. Teachers' perceptions of their experiences both in surveys and interviews demonstrated high satisfaction with the quality and usability of the PD, including positive beliefs related to the social capital elements of tie quality, depth of interaction, and access to expertise. Further transactivity analyses of their interactions in course discussions showed higher levels of collaborative discourse resulting from prompts that specifically targeted the exchange of information over those that asked teachers to reflect about their content understanding or their classroom practice. Implications for this design for asynchronous online PD approaches to reach more teachers are discussed.